Communication systems having multiple transmitter stations disposed alongside a road for transmitting data to passing vehicles have been proposed in JP-A-2001-268625 (U.S. application Ser. No. 09/804,475 filed on Mar. 13, 2001) now U.S. Pat. No. 6,909,893. Improvements in communication protocols have also made it possible to transmit large amounts of data in a short time. This has made it feasible to transmit video signals and other types of information containing large amounts of data from the roadside transmitter stations to passing vehicles.
Despite improvements in data communication protocols, data transmission needs exceed the transmission capacity of the protocol. In order to transmit large amounts of data to vehicles travelling at high speeds, it is therefore necessary to install multiple transmitter stations alongside the road so that the vehicles receive all of the transmitted data by collecting small blocks of the data from plural transmitter stations.
One proposed system for transmitting data in segments from plural transmitter stations is shown in FIG. 12A. A network management server 200 first saves the data to be distributed to the plural transmitter stations in a memory, and then sends the data via a communication line to a switch 201. This communication line could be a wired or wireless path. Data traffic flow along this communication line is controlled by the network management server 200 segmenting or dividing the transmission data into packets for transmission to each transmitter station, and addresses each packet to a particular transmitter station 202. As shown in FIG. 12B, each packet includes a header (H1 to H4) and data (D1 to D4). The header indicates address to be directed. The switch 201 then reads the address information in each packet to distribute each data packet to the proper transmitter station 202.
This network server 200 executes processing shown in FIG. 13. The first task after server operation starts is to receive and store data required for transmission a memory of the network management server 200 (step 211). It then determines the size of the packets, that is, how much data to send to each of the plural transmitter stations 202 (step 212). The packet address identifying the transmitter station to which the packet will be sent is also determined when determining the packet size at step 212. The required packet size is determined according to the transmission capacity of the transmitter station 202 and the importance of the data.
A data block of the size determined for transmission to one transmitter station 202 is then extracted (step 213). If the packet is to be sent to transmitter station TR1, for example, the address of transmitter station TR1 is added to the packet as the header H1 (step 214). The data D1 is then sent as a transmission packet to the switch 201 (step 215). Step 216 then detects if there is any remaining data to be sent. If there is (YES), steps 213 to 215 repeat to generate and send the next data packet to the next transmitter station, such as transmitter station TR2. When step 216 determines that all of the data has been distributed to the transmitter stations, operation ends.
Packets containing address information are thus input from the network management server 200 to the switch 201, and processed by the switch 201. The switch 201 receives the packet, reads the address from the input packet header, and send the data to the addressed transmitter station 202. More specifically, as shown in FIG. 14, when the switch 201 operation starts, it receives a packet and stores the received packet to a local memory (step 221). The switch 201 then reads and interprets the address of the transmitter station to which the packet must be sent (step 222). The switch 201 thus determines that the received packet is to be sent to transmitter station TR1 in this example (step 223), and then sends the packet to the addressed transmitter station (step 224). If there are four transmitter stations TR1 to TR4, for example, this operation repeats for each of the four transmitter stations 202 to send data to all of the transmitter stations 202, and operation then ends.
The network management server 200, switch 201 and transmitter stations 202 operates as shown in FIG. 15. The network management server 200 and switch 201 distribute the data received from a central higher-order server 204 to the transmitter stations 202, and the transmitter stations 202 simply retransmit the received data. More specifically, the network management server 200 must store data received via a communication network, allocate the data to particular transmitter stations, and generate and address transmission packets. The switch 201 must then read all of the received data and distribute the data to each of the addressed transmitter stations 202.
The burden of allocating data is thus concentrated on the management server 200 and switch 201. Operation of the network management server 200 and switch 201 thus becomes a bottleneck making it difficult to transmit large amounts of information at high speeds using transmitter stations 202 installed along an expressway or other road where the vehicles travel at high speed, particularly when there are many lanes and when large amounts of data such as video images are distributed.